layout: ensure Spacer obeys constraints

This commit ensures that the Spacer type doesn't break layouts
by ignoring when its min constraints require it to be larger or
its max constraints require it to be smaller.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

layout/layout.go

@@ -211,10 +211,10 @@ type Spacer struct {
 
 func (s Spacer) Layout(gtx Context) Dimensions {
 	return Dimensions{
-		Size: image.Point{
+		Size: gtx.Constraints.Constrain(image.Point{
 			X: gtx.Dp(s.Width),
 			Y: gtx.Dp(s.Height),
-		},
+		}),
 	}
 }